The invention relates to a data acquisition apparatus and method. The invention relates in particular to an apparatus and a method for seismic data acquisition, the apparatus being intended to form a node of a seismic sensors network. More accurately, the present invention relates to a seismic data acquisition apparatus being made up of one digital signal processing mean connected to a digital seismic sensor or to an analog seismic sensor associated with an analog/digital converter both producing a series of digital seismic samples or ground displacement measurements. All the nodes must be synchronized so that they all have the same sampling time.
A person skilled in the art will be aware of numerous data acquisition apparatuses and methods intended to be implemented within a data acquisition network. In the case where the data acquisition network is formed of an assembly of seismic sensors, these devices are conventionally and usually constituted by sensors linked together in groups, by wires or cables, to a “node” of the network. These nodes are generally organized in groups around a “concentrator”, and a group of nodes forms, with its concentrator, a subnetwork, the links between the concentrator and the nodes also being wire links. The concentrators locally manage the subnetwork of nodes, provide the power supply and centralize the data. The concentrators are connected to a local computer network, likewise a wire network, to which is connected a Central Unit which drives the assembly and records the data originating from the subnetworks. These conventional solutions are well known to the person skilled in the art and will not be described in detail hereinbelow.
The person skilled in the art has also proposed wireless data acquisition networks which have the obvious advantage of avoiding the use of numerous cables. In the case of seismic applications, these networks are also formed of nodes, of concentrators and of a Central Unit according to an organization in accordance with what is described hereinabove, the communication between these various elements of the network being performed by RF in radio mode. A classical technique is to transmit to all of the nodes only one sampling reference clock. Each node extracts and maintains this sampling clock thanks to a phase-locked loop system (PLL) controlling a local oscillator. The use of a VCXO (voltage controlled crystal oscillator) controlled by a PLL has a high cost and a high energy consumption and involves strong hardware requirements for transmitting the clock.
Another data acquisition apparatus is known from document GB-2 428 799. It comprises an analog/digital converter sampling at an imperfect frequency provided by a local clock data acquired by a sensor thus providing a series of sampled and dated data, a time-stamping module for measuring the frequency error of the local clock by determining the sampling dates according to a universal time and comprising: a satellite-based positioning system including a reference clock, the time-stamping module being able to gauge the local clock to the reference clock of the satellite-based positioning system, means for turning off the satellite-based positioning system once the gauging of the local clock has been performed, a resampling module for correcting the sampled and dated data. The resampling module is able to generate, on the basis of a series of the sampled and dated data, a series of resampled and redated data, that is to say in which the phase error and sampling frequency error of the series of data are both compensated. The resampling module comprises a first memory storing the series of data dated and sampled at a frequency corresponding to the frequency of the local clock, and a second memory storing the sampling dates determined according to the universal time. The resampling module comprises a resampler linked on the one hand to a first memory containing the series of sampled and dated data and linked on the other hand to an interpolation filter, said interpolation filter itself being linked to a second memory containing the sampling dates determined according to the universal time and linked also to a reference filter through which the interpolation filter is dynamically calculated for each sample of a series of samples to be calculated, said interpolation filter thus generating the corrective interpolation coefficients making it possible to generate the series of resampled data within the resampler.
Said known apparatus has revealed to be difficult and expensive to be made in practice. It has the disadvantage of requiring a high computing load. The interpolation filter has a high degree i.e. a high number of coefficients and has to be recalculated for each sample. It involves a strong constraint in memory size, a high number of calculations in the short time allowed by the time elapsed between successive samples, and thus a high consumption in energy.
An objective of the present invention is therefore to propose an apparatus and a method to alleviate the above-mentioned inconvenience of the state of the art. Another objective of the present invention is to propose an apparatus and a method making it possible to obtain accurate acquisition data while minimizing energy consumption, computing load and memory load.
These objectives are achieved within the framework of the present invention by virtue of a data acquisition apparatus, comprising:                a first means providing a series of first digital sampled and dated data (X) at an imperfect sampling frequency (FE) provided by a first local clock (2),        a second gauging means (50) for measuring the frequency error of the first local clock (2) in view of a second reference clock (6),        a third means for correcting the first data based at least on the measured frequency error,        
characterized in that                the first means (3) comprises sigma-delta modulation means (3) arranged to produce said series of first digital sigma-delta modulated data (X) sampled at said first imperfect sampling frequency (FE) provided by said first local clock (2),        the third means comprises at least an interpolation means (4) to produce second digital data (Y) based on at least an interpolation of said first series of data samples (X) and compensating the measured frequency error (FD), and at least a decimation filter (7) for producing third digital data (A) based on said second digital data (Y).        
According to an embodiment of the invention, the first means comprises at least an analog sensor (102) producing analog measurement data and the sigma-delta modulation means (3) is in the form of a sigma-delta analog—digital converter (3) converting said analog measurement data of the sensor (102) into said series of first digital sigma-delta modulated data (X). According to an embodiment of the invention, the first means comprises at least a digital sensor having said sigma-delta modulation means for producing said series of first digital sigma-delta modulated data (X). According to an embodiment of the invention, the apparatus comprises a control means (42) arranged to set said interpolation based on at least said measured frequency error (FD). According to an embodiment of the invention, the interpolation means (4) has an interpolation function (F) having a preset fixed degree and at least a parameter (C0, C1, C2), the third control means (42) comprises a means arranged to set said parameter (C0, C1, C2) of the interpolation function (F) based on at least said measured frequency error (FD). According to an embodiment of the invention, the degree of the interpolation function (F) is lower than six.
According to an embodiment of the invention, the interpolation function (F) is linear and the control means (42) sets the interpolation function (F) according to interpolation formula for current first sample Xn:Yn═C0·XnC1·Xn−1,whereinC0=Pn C1=1−Pn wherein Xn−1 and Xn are respectively successive first samples (X),Yn is the interpolated third data (Y) for Xn−1 and Xn,wherein the control means (42) calculates the value Pn of an interpolation coefficient (P) according to formula:Pn=Pn−1+FD wherein Pn−1 and Pn are respectively successive values of the interpolation coefficient P and FD is the frequency drift.
According to an embodiment of the invention, the interpolation function (F) has a degree equal to two and the control means (42) sets the interpolation function (F) according to interpolation formula for current first sample Xn:Yn=C0·Xn+C1·Xn−1+C2·Xn−2,wherein
            C      0        =                            1          2                ⁢                  P          n          2                    -                        1          2                ⁢                  P          n                    +              1        8                        C      1        =                  3        4            -              P        n        2                        C      2        =                            1          2                ⁢                  P          n          2                    +                        1          2                ⁢                  P          n                    +              1        8            wherein Xn−2, Xn−1 and Xn are respectively successive first samples (X),Yn is the interpolated third data (Y) for Xn−2, Xn−1 and Xn,wherein the third control means (42) calculates the value Pn of an interpolation coefficient (P) according to formula:Pn=Pn−1+FD wherein Pn−1 and Pn are respectively successive values of the interpolation coefficient P and FD is the frequency drift.
According to an embodiment of the invention, the control means (42) comprises a means (43, 44) to check whether according to formula Pn=Pn−1+FD the value Pn of the interpolation coefficient (P) is higher than or equal to zero and lower or equal to one, and as long as the interpolation coefficient is higher than or equal to zero and is lower or equal to one, the control means (42) sets the interpolation function (F) according to interpolation formula for Xn with the value Pn of the interpolation coefficient (P). According to an embodiment of the invention, the control means (42) comprises a means (44) to check whether according to formula Pn=Pn−1+FD the value Pn of the interpolation coefficient (P) is higher than one for current first sample Xn, and if according to formula Pn=Pn−1+FD the value Pn of the interpolation coefficient (P) calculated for current first sample Xn is higher than one, then the control means (42) does not use the corresponding interpolated value Yn calculated according to interpolation formula as third data (Y) for current first sample Xn with Pn=Pn−1+FD, but produces as third data associated with current first sample Xn a value Yn calculated according to interpolation formula for current first sample Xn with the value Pn of the interpolation coefficient calculated according to Pn=Pn−1+FD−1.
According to an embodiment of the invention, the control means (42) comprises a forecast means (43) to check whether according to formula Pn+1=Pn+FD the next value Pn+1 of the interpolation coefficient (P) for current first sample Xn is lower than zero, and if according to formula Pn=Pn−1+FD the value Pn of the interpolation coefficient (P) calculated for current first sample Xn is lower than zero, then the control means (42) produces as third data (Y) the corresponding interpolated value Yn calculated according to interpolation formula for current first sample Xn with Pn=Pn−1+FD and produces as third data (Y) another value calculated according to interpolation formula for current first sample Xn with the value Pn of the interpolation coefficient calculated according to Pn=Pn−1+FD+1. According to an embodiment of the invention, said series of first digital sigma-delta modulated data (X) has a first preset bit resolution (BR1) of at least one and of lower or equal to four, the second data (Y) has said first sampling frequency (FE) and a second preset bit resolution (BR2) higher than the first bit resolution (BR1), and the third digital data (A) has a third preset sampling frequency (F3) lower than the first sampling frequency (FE) and a third preset bit resolution (BR3) higher than the first bit resolution (BR1). According to an embodiment of the invention, the value of the first local clock frequency FE is preset to cause an oversampling of the signal provided by at least a sensor according to an oversampling rate OR higher than one, wherein the oversampling rate OR is defined as being equal to:OR=FE/2·FU.wherein FU is the highest useful frequency of said signal provided by said sensor.
These objectives are achieved within the framework of the present invention also by virtue of a data acquisition method carried out by calculation means, comprising the steps of:                providing a series of first digital sampled and dated data (X) at an imperfect sampling frequency (FE) provided by a first local clock (2),        measuring the frequency error of the first local clock (100) in view of a second reference clock (6) by gauging the first local clock (100) in view of the second reference clock (6),        correcting the first data based at least on the measured frequency error,        
characterized in that                said series of first digital sigma-delta modulated data (X) sampled at said first imperfect sampling frequency (FE) provided by said first local clock (2) is produced by a sigma-delta modulation (3),        said series of first digital sigma-delta modulated data (X) is interpolated to produce second digital data (Y) in order to compensate the measured frequency error (FD),        said second digital data (Y) is decimated for producing third digital data (A).        